1. Field of the Invention
The present invention relates to a measurement system and a method for establishing the radius of curvature of the cornea, the refraction of an eye or the internal pressure of the eye. Additionally, the invention relates to a method and a system for adjusting the internal eye pressure.
2. Description of the Related Art
Establishing the refraction of the eye during surgery, i.e. intraoperatively, is of importance in a number of types of eye surgery. Examples thereof include the working of the corneal surface by means of laser beams to correct defective vision and cataract operations, in which the lens of the eye is replaced by an artificial lens. Methods and a device for determining the refraction of an eye are for example described in EP 0 563 604 A1 and EP 1 338 238 A2.
Irrespective of which method is used to establish the refraction, a problem always remains in ensuring that the refraction is also determined on the visual axis of the eye. This axis is not readily visible on the eye and sometimes differs significantly from the optical axis of the eye. This problem can be avoided by offering the eye a fixing marking, onto which the gaze can be fixed. However, this requires an active action of the patient and is only possible in the case of so-called eye drop anesthetics, in which the patient retains the ability to fixate objects and move the eye, provided the eye was not fixed by the surgeon. By contrast, if the patient is under general anesthesia or the patient's eye was anesthetized in a retrobulbar fashion, i.e. the anesthetic was injected behind the eyeball, the patient can no longer actively move the eye and can no longer actively fixate the eye either. The alignment of the visual axis of the eye, the refraction of which should be measured, is then completely arbitrary with respect to the measurement arrangement. If the measurement cannot be guaranteed to be taken on the visual axis, the measurement result will be afflicted by smaller or larger errors. By way of example, this can be redressed by measuring the eye before the operation and applying corresponding markings onto the eye of the patient. Such markings are used particularly with regard to the axis alignment of toric intraocular lenses within the scope of a cataract operation.
DE 197 02 335 has disclosed a device with a pulsed laser, which is suitable for corneal ablation. The eye is irradiated with infrared beams for beam tracking. A camera records images, generated by the infrared beams, of the eye, more particularly of the pupil, in the form of a light/dark contrast. A computer connected to the camera then establishes e.g. the centroid of the dark field, i.e. of the pupil, or the edge of the pupil, and so the eye movement could be established independently of the current pupil diameter. Although this can establish the position of the eye, it cannot establish the visual axis.
U.S. Pat. No. 6,299,307 B1 describes an eye-tracking device for laser surgery, in which the interface between the white sclera and the colored iris is established for determining the position of the eye. However, the visual axis cannot readily be calculated therefrom. Furthermore, this document explains that Purkinje images can be used to establish the eye position. However, it notes that the optical quality of the eye deteriorates temporarily during eye surgery, and so the Purkinje images go out of focus and this makes a precise determination of the eye position very difficult.
US 2006/0247659 A1 describes a surgical microscope system and a method for performing eye surgery. The system comprises an eye tracker, which analyzes images of the eye in order to establish the inner and outer boundary of the iris, the sclera and the eyelids. Image-processing software then establishes the largest connected dark region, which in practice corresponds to the pupil. The eye-tracking is then undertaken by determining the geometric center of the largest connected region. This method does not allow the visual axis to be determined.
U.S. Pat. No. 5,474,548 describes a method for establishing an unambiguous, machine-independent reference system for the eye. This method can be used in particular within the scope of eye surgery. It allows the alignment of the visual axis of the patient with regard to the optical axis of an ophthalmologic instrument, which is used for laser surgery or for diagnostic measurements. The method comprises moving the eye in the lateral direction until the images of a first and a second reference marking, which are located along the optical axis of the ophthalmologic instrument at different distances from the eye, align. During the treatment, the patient fixates the offered markings. This does not therefore allow the determination of the visual axis when the patient is under general anesthesia or in the case of retrobulbar anesthesia.
A further difficulty in the intraoperative refraction measurement lies in the fact that the internal eye pressure of the eye currently undergoing surgery can deviate significantly from the natural internal eye pressure, for example as a result of the injection of viscoelastic substances into the anterior chamber of the eye (to prevent the eye from collapsing during surgery). This modified internal eye pressure can lead to a change in the radius of the cornea and hence to a wrong refraction measurement.
It is therefore an object of the present invention to provide a method and a measurement system for establishing the refraction of an eye, which method and measurement system more particularly are also suitable for measuring the refraction intraoperatively, when the patient is under general anesthesia or in the case of retrobulbar anesthesia.
It is a further object of the invention to provide an advantageous measurement system and an advantageous method for establishing the radius of curvature and/or the internal pressure of an eye, which, more particularly, can also be used within the scope of a method for the intraoperative measurement of the refraction of an eye.
An additional further object of the present invention is to provide a system and a method for adjusting the internal pressure of an eye.